Earlier this week, Tomas Nunuk of Bratislava, Slovakia uploaded a video onto YouTube of himself and a friend hiking near the border of Slovakia and Poland. When the hikers came across Veľké Hincovo pleso, a lake frozen over 250 days out of the year, they decided to walk across. The video isn't just amazing because of the picturesque view of the High Tatras Mountains in the background. It is amazing because though the lake was completely frozen, the ice was as pure and clear as if they were walking on glass.
You’ll want to go full screen and check this video out in high-definition to fully appreciate how insanely clear the ice is:
In fact, the clarity of the ice was so exceptional, many commenters on YouTube thought the video had been faked. After all, the ice typically seen on the top of bodies of water is white. So what’s the deal? Is it actually possible for the Velke Hincovo Pleso to have a clear, glass-like appearance, or is this all a clever ruse?
The ice in the video is an example of congelation ice, which is also known as black ice because of an ability to see through to the color of the water underneath. The congelation ice forms underneath an existing layer of ice, building off of the bottom. When the top layer is melted, the crystal-clear layer is revealed. The incredible clarity of the ice is a result of freezing with no air bubbles, snow, and impurities present. When impurities or air bubbles are trapped in ice, it causes the light to scatter as it travels through and results in a cloudy appearance.
The purest congelation ice forms slowly, creating large crystals. This ice is fairly strong, though you can almost hear the sound of ice straining underneath the weight of Tomas and his friend. Veľké Hincovo pleso has a maximum depth 53 meters (174 feet), so the pair were very lucky the ice held out!
[Hat tip: io9]